The present invention relates to implantable medical devices, and more particularly to a voltage converter for use within an implantable microstimulator, or similar implantable device, that uses an RF-powering coil instead of capacitors to provide a voltage step-up and step-down function.
Many implantable medical devices, such as neural stimulators, sensors, and the like, utilize a battery as a primary source of operating power. Other types of implantable devices, such as cochlear stimulators, rely on the presence of an alternating magnetic field to induce an ac voltage into the implantable device, where the induced voltage is thereafter rectified and filtered in order to provide the primary operating power for the device. In both types of devices—a battery-powered device or an RF-powered device—there is a frequent need to derive other operating voltages within the device from the primary power source. That is, there is a frequent need to step up the voltage of the primary power source to a higher voltage in order to, e.g., generate a high stimulation current or for some other purpose. Similarly, in some devices, there is also a frequent need to step down the voltage of the primary power source to a lower voltage for use in certain types of circuits in order to, e.g., conserve power.
In order to perform the voltage step-up or step-down function, it is known in the art to use a charge-pump voltage converter circuit. Charge pump circuits typically rely on a network of capacitors and switches in order to step up and step down a primary voltage source. For example, in order to step up a primary voltage source, a network of, e.g., four capacitors, may be connected in parallel through a switching network and maintained in the parallel connection configuration until each capacitor charges to the voltage of the primary power source. The voltage of the primary power source is, e.g., the battery voltage (where a battery is used as the primary power source). Once thus charged, the capacitors are switched so that they are connected in series, thereby effectively creating a voltage across the series connection that is four times the voltage of the primary voltage source. The charge associated with this higher voltage may then be transferred to another capacitor, e.g., a holding capacitor, and this process (or charging parallel-connected capacitors, switching them in series, and then transferring the charge from the series connection to a holding capacitor) is repeated as many times as is necessary in order to pump up the charge on the holding capacitor to a voltage that is four times as great as the voltage of the primary power source.
While charge-pump circuits have proven effective for performing step up and step down functions, such circuits require a large number of capacitors, which capacitors may be quite large and bulky. Charge pump circuits that use large numbers of bulky capacitors are not well suited for implantable medical devices that must remain very small. Moreover, charge pump circuits tend to be relatively slow and inefficient in operation. What is needed, therefore, is a voltage converter circuit that is able to perform the step up or step down function, efficiently, quickly, and without having to rely on the use of a large number of bulky capacitor/s.